Field help (PRIRENPPC) 

The Reason is a local menu code associated with a price list rule.
During the price list search, this reason is assigned to each value of the document line (quote, order, delivery notes, invoice etc.) using this price list.
This is used for:

A dedicated setup makes it possible to define a default value for the reasons when the price is modified manually.

SEEREFERTTO See the documentation on the Price list reasons for more information.